Count your strokes for a length and try to decrease the number on the next length
by gliding in the propulsive phase (the underwater part) of the stroke
Ensure your propelling hand pushes all the way back during the stroke to push you forward in the water.
Swimming with one arm ahead of you and one arm to your side, propel yourself for a length with only your kick. Focus on being horizontal in the water, keeping a soft leg and kicking from the hips.
As it sounds, swim a length with your hands curled in fists - this will encourage the use of the forearm to catch water and help propulsion. It's one of the best drills for learning how to feel the water you are trying to move through.